1. Field of the Invention
The present invention relates to a method of making a high definition chevron type magnetoresistive ("MR") sensor, and more particularly to a method of sputtering a chevron shaped MR sensor with high depth control, with planar walls that meet at 90.degree., and with no fencing upon removal of a patterning mask.
2. Description of the Related Art
An MR read head is employed for sensing magnetic fields on a magnetic storage medium, such as a longitudinally moving magnetic tape of a tape drive. In such a tape drive, the MR head is mounted on a support, while guide rollers bias the tape into contact with a head surface of the read head. When the tape moves longitudinally, the read head senses magnetic fields in longitudinally extending tracks on the magnetic tape An actuator is connected to the support for moving the read head transversely across the tape, to selected information tracks. A write head is typically combined with a magnetoresistive (MR) read head to form a combined or merged MR head. With this arrangement the merged MR head is also capable of writing information signals on selected longitudinally extending tracks as positioned by the actuator. Typically the write head writes tracks at a certain width, while the read head reads a narrower track width. In the art this is referred to as "write-wide, read-narrow".
An MR head has an MR sensor sandwiched between first and second gap layers that, in turn, are sandwiched between first and second shield layers. The MR sensor includes multiple thin film layers. The most important layer is an MR stripe; the other layers are for biasing and capping. The spacing between the first and second shield layers determines the linear read resolution of the MR head with respect to an information track. The MR stripe, which may be NiFe, is typically elongated, with its length aligned parallel to the head surface and perpendicular to the movement of the tape. By shape anisotropy this elongation establishes an easy magnetic axis along the length of the MR stripe. Thus, without some biasing scheme, the magnetic moment of the MR stripe will be directed along its lengthwise direction.
First and second leads may be connected to opposite lengthwise ends of the MR sensor for conducting a sense current through the MR stripe. The spacing between the leads establishes the active region of the MR sensor, which defines the sensor's track width. Sense current is provided by channel electronics, which may also be referred to as processing circuitry. When sense current is conducted through the MR stripe, resistance changes in the MR stripe cause proportional changes in potential across the stripe. These potential changes are then processed to produce playback signals corresponding to data signals stored on the magnetic tape. It is important that the MR sensor be constructed with predetermined conductivity to satisfy the design of the channel electronics. Conductivity limits are exceeded when the MR sensor is constructed with too much or too little conductive material.
A typical type of MR sensor is an anisotropic MR (AMR) sensor. The transfer function of the AMR sensor varies by cos.sup.2 .alpha., where .alpha. denotes the angle between a magnetization direction and a current-density vector. The transfer function is a plot of the resistance change of the MR sensor as a function of the strength of an applied field signal from the tape, and can be shown as a curve (transfer curve) on a graph. When the direction of the magnetic moment of the MR stripe and the direction of the sense current in the MR sensor are parallel, the MR stripe has maximum resistance; when these directions are perpendicular, the MR stripe has minimum resistance. It is desirable that an AMR sensor operate within a linear portion of a bell-shaped transfer curve. This is accomplished by appropriately positioning the direction of the magnetic moment of the MR stripe 45.degree. to a plane of the head surface. Assuming that positive and negative field signals from the moving magnetic tape are equal, then each of the clockwise and counterclockwise rotations of the magnetic moment, the positive and negative changes in magnetoresistance of the MR stripe, and the positive and negative signal responses will be equal.
One way to bias the magnetic moment of the MR stripe at 45.degree. is to provide the MR sensor with a soft adjacent layer (SAL) adjacent to the MR stripe, but separated therefrom by an insulation layer. When the sense current is conducted through the MR stripe, a transverse field is applied to the SAL and the SAL, in turn, applies a transverse field to the MR stripe which rotates the magnetic moment of the MR stripe to the 45.degree. angle.
Another way to obtain the desired bias angle is to employ shape anisotropy to establish the direction of the magnetic moment of the MR stripe along the desired bias angle. This scheme is employed by a chevron type MR sensor which consists essentially of only the MR stripe. The chevron type MR sensor has a plurality of elongated ridges that are uniformly spaced from one another. Between the ridges are elongated trenches. These ridges and trenches are slanted to the plane of the head surface by some angle, such as 45.degree.. This arrangement has the appearance of multiple chevrons, as seen in cross-section, such as a head surface view. The result is that the easy axis of the MR stripe is established along the angle of the chevron structure or close thereto instead of being parallel to the head surface. A SAL is not employed in this scheme. When sense current is conducted through the MR stripe, the current is directed parallel to the head surface and the magnetic moment is directed at substantially 45.degree. to the head surface. This provides the aforementioned desirable bias angle for the operation of the MR sensor.
Typically, an MR sensor has first and second surfaces that are perpendicular to the head surface. Each of the first and second surfaces is configured with the ridges to provide a ribbed structure. The ridges on the first surface are positioned opposite the trenches on the second surface and the ridges on the second surface are positioned opposite the trenches on the first surface. Between the ridge structures, an intermediate portion of the MR sensor connects the ridges together.
Each ridge is bounded by first and second side walls, a flat surface at the top of the ridge and the intermediate portion. Each trench between a respective pair of ridges is bounded by the side walls of the ridges and a flat surface at the bottom of the trench. In order to promote an orderly shape anisotropy of the MR sensor it is necessary that the side walls be planar and perpendicular to the flat surfaces of the ridges and the side walls. As stated hereinabove, the MR sensor is sandwiched between the first and second gap layers. These layers are typically constructed of alumina (AL.sub.2 O.sub.3). Accordingly, the ridges and trenches of the first surface of the MR sensor interfacially engage trenches and ridges respectively in the first gap layer and the ridges and trenches of the second surface of the MR sensor interfacially engage trenches and ridges of the second gap layer.
The present method of making the chevron type MR sensor typically results in poorly formed chevron structures. First a photoresist mask is spun on the first gap layer and patterned by light followed by dissolving the exposed portions to provide elongated spaced apart openings that are slanted at the appropriate angle to the head surface. Next, ion beam milling is employed to mill elongated trenches in the first gap layer that are spaced apart by non-milled elongated top surfaces therebetween. These trenches and surfaces are slanted to the head surface. The photoresist layer is then removed and MR material is sputtered into the trenches and on top of the top surfaces. This provides each of the first and second surfaces of the MR sensor with the chevron structure. The second gap layer is then deposited on the chevron structure of the second surface of the MR sensor.
Unfortunately, the step of ion milling the first gap layer results in the first gap layer having poorly formed side walls, unreliable depths and fencing. The side walls have various slopes with respect to the top surfaces of the first gap layer, the depths are too deep upon overmilling, or too shallow upon undermilling, and fencing is caused by redeposition of the milled material (redep) on the side walls. Since the redep typically sticks up above the side wall it appears as a fence. Next, during the step of depositing the MR material, the MR material replicates the shape of the first gap layer and has sloping side walls, bottoms that are too deep or too shallow and fencing. The manufacturing yield has been extremely low with the present method because of process variations in the amount of conductive material in the chevron shaped MR sensor. As stated hereinabove, the amount of conductive material must be precise to satisfy the requirements of the channel electronics. Accordingly, there is a strong felt need to provide an improved method of making chevron structures for AMR read heads.